Where's My Roof? Why Northern Football Stadiums Go Topless

On Sunday, the Denver Broncos and Seattle Seahawks will square off at MetLife Stadium in New Jersey under the open sky. It's the first time in football history the Super Bowl has been played in an open stadium in a cold-weather city.

The decision has drawn plenty of detractors, and fans are glued to forecasts in advance of the big game. But why is MetLife Stadium open anyway? And why do so few cold-weather cities spring for a stadium with a roof?

The reason, it turns out, has only a little to do with football. While weather matters, stadium designs are chosen based on potential uses outside the sport. [In Photos: The Coldest Places on Earth]

Stadium statistics

First, the numbers. There are 31 official NFL stadiums. We divided them into north and south based on the traditional boundary between the regions — the Mason-Dixon line. Western stadiums were a bit harder to categorize by this criteria, so Live Science used the latitude of the Mason-Dixon line as a marker.

This had some odd effects, such as lumping the O.co Coliseum in Oakland, Calif., and the 49ers' Levi's Stadium in Santa Clara, Calif., in with Deep South stadiums like New Orleans' Superdome. Nevertheless, the western stadiums that didn't fit nicely into preconceived notions of North and South were all open stadiums, so their categorization didn't change the final picture. (Yes, arguably too much thought went into this. Football is serious business.)

Using these criteria, there are 11 open football stadiums in the North and 12 in the South. The real difference appears when looking at closed and retractable-roof stadiums. There is only one closed stadium north of the Mason-Dixon line: Detroit's Ford Field. The South also boasts only one retractable-roof stadium, Lucas Oil Stadium in Indianapolis, Ind. [In Photos: NFL Stadiums That Have Hosted the Super Bowl]

In the South, there are three domed stadiums (the Mercedes-Benz Superdome in New Orleans, the Georgia Dome in Atlanta, and the Edward Jones Done in St. Louis). There are also three southern retractable-roof stadiums (Houston's Reliant Stadium, Arizona's University of Phoenix Stadium, and AT&T Stadium in Arlington, Texas, which is home to the Dallas Cowboys).

Technical challenge

To find out why teams and cities choose an open stadium versus a retractable-roof or closed dome, we turned to Walter P. Moore, the Houston-based engineering firm that engineered the moveable roofs of University of Phoenix Stadium, AT&T Stadium, Lucas Oil Stadium and Reliant Stadium.

The choice, said Mark Waggoner, a principal at the firm, boils down to a few issues: cost, uses other than football, and, yes, weather.

Putting a roof over a stadium is both an engineering and financial challenge. Building an unsupported structure that spans some 600 to 800 feet (180 to 245 meters) is more like building a bridge than a ceiling, Waggoner told Live Science.

"It's either a lot of steel or a lot of cables," he said.

Domes are challenging, because they require a lot of temporary support during construction, Waggoner said. A retractable roof is another leap.

"It's a challenge just to build a basic roof over the stadium, so when we put a hole in it and start moving a piece of it, it becomes that much more complicated," Waggoner said.

Retractable roofs use different methods based on the slope of the building, but generally panels roll along rails or geared tracks, powered by electric motors, he said. The popularity of these buildings has taken off since the late 1990s, however, he said — and the four retractable-roof football stadiums have all opened in the last 12 years. [10 Technologies That Will Transform Your Life]

All this roof technology comes with a price tag. A retractable roof adds between $100 million and $150 million to a project over an open stadium, Waggoner said, and between $25 million and $40 million over the cost of a closed, fixed-roof stadium.

Choosing a roof

So why swallow the extra bill? Basically, an enclosed stadium gets more use.

"The football games have probably the least to do with that decision," Waggoner said. If a municipality wants the stadium for use in concerts, rodeos or other year-round events, a closed building is the way to go. Purists who see football as an outdoor game may be mollified by the open-stadium option of a retractable roof.

MetLife Stadium was originally envisioned as a retractable-roof stadium, but the owners of the New York Giants and New York Jets, who share the facility, balked at the cost, according to a report about the stadium. With two teams playing on the field, the stadium gets more football-related use than many NFL facilities, noted Lee Slade, a senior principal at Walter P. Moore.

As for why more southern climes spring for enclosed facilities, the difference may be weather. As much as fans are grumbling about needing to bundle up for this year's Super Bowl, it's easier to pile on the down coats in winter weather than to stay cool in high heat, humidity and sunshine.

"Certainly in the South, the heat plays a factor in the patron comfort during games," Waggoner said.

Labor costs in New Jersey would likely drive the price of building a roofed stadium beyond even what it would be in the South, Waggoner said. And designing for snow requires more steel (which means more cash), as the weight of a snowfall can double the load a stadium roof needs to support.

In the future, retractable, multi-use stadiums will probably become more and more popular, Waggoner said. But engineers may experiment with moving walls and other designs, especially because artificial turf has removed the need for direct sunlight on the playing field.

"I think we'll see more roofs that aren't just traditional … but rather are kinetic and moving in ways that affect fan comfort, but aren't necessarily just trying to get sunlight directly on the field," he said. "I think we're see more lightweight systems, inflatable fabrics and things that like that may cover spaces cost-effectively."